Bioreactor using acoustic standing waves

ABSTRACT

A perfusion bioreactor includes at least one ultrasonic transducer that can acoustically generate a multi-dimensional standing wave. The standing wave can be used to retain cells in the bioreactor, and can also be utilized to dewater or further harvest product from the waste materials produced in a bioreactor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/432,888, filed Feb. 14, 2017, which is a continuation of U.S. patentapplication Ser. No. 15/238,624, filed Aug. 16, 2016, which is acontinuation-in-part of U.S. patent application Ser. No. 14/175,766,filed Feb. 7, 2014, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/761,717, filed on Feb. 7, 2013, and which isalso a continuation-in-part of U.S. patent application Ser. No.14/026,413, filed on Sep. 13, 2013, which is also a continuation-in-partof U.S. Ser. No. 13/844,754, filed Mar. 15, 2013, which claimed thebenefit of U.S. Provisional Patent Application Ser. No. 61/708,641,filed on Oct. 2, 2012, and of U.S. Provisional Patent Application Ser.No. 61/611,159, filed Mar. 15, 2012, and of U.S. Provisional PatentApplication Ser. No. 61/611,240, also filed Mar. 15, 2012, and of U.S.Provisional Patent Application Ser. No. 61/754,792, filed Jan. 21, 2013.These applications are incorporated herein by reference in theirentireties.

BACKGROUND

The field of biotechnology has grown tremendously in the last 20 years.This growth has been due to many factors, some of which include theimprovements in the equipment available for bioreactors, the increasedunderstanding of biological systems and increased knowledge as to theinteractions of materials (such as monoclonal antibodies and recombinantproteins) with the various systems of the human body.

Improvements in equipment have allowed for larger volumes and lower costfor the production of biologically derived materials such as recombinantproteins. This is especially prevalent in the area of pharmaceuticals,where the successes of many types of new drug therapies have beendirectly due to the ability to mass produce these materials throughprotein-based manufacturing methods.

One of the key components that is utilized in the manufacturingprocesses of new biologically based pharmaceuticals is the bioreactorand the ancillary processes associated therewith. An area of growth inthe bioreactor field has been with the perfusion process. The perfusionprocess is distinguished from the fed-batch process by its lower capitalcost and higher throughput.

In the fed-batch process, a culture is seeded in a bioreactor. Thegradual addition of a fresh volume of selected nutrients during thegrowth cycle is used to improve productivity and growth. The product isrecovered after the culture is harvested. The fed batch bioreactorprocess has been attractive because of its simplicity and also due tocarryover from well-known fermentation processes. However, a fed-batchbioreactor has high start-up costs, and generally has a large volume toobtain a cost-effective amount of product at the end of the growthcycle. After the batch is completed, the bioreactor must be cleaned andsterilized, resulting in nonproductive downtime.

A perfusion bioreactor processes a continuous supply of fresh media thatis fed into the bioreactor while growth-inhibiting byproducts areconstantly removed. The nonproductive downtime can be reduced oreliminated with a perfusion bioreactor process. The cell densitiesachieved in perfusion culture (30-100 million cells/mL) are typicallyhigher than for fed-batch modes (5-25 million cells/mL). However, aperfusion bioreactor requires a cell retention device to prevent escapeof the culture when byproducts are being removed. These cell retentionsystems add a level of complexity to the perfusion process, requiringmanagement, control, and maintenance for successful operation.Operational issues such as malfunction or failure of the cell retentionequipment has previously been a problem with perfusion bioreactors. Thishas limited their attractiveness in the past.

BRIEF DESCRIPTION

The present disclosure relates, in various embodiments, to a system forproducing biomolecules such as recombinant proteins or monoclonalantibodies, and for separating these desirable products from a cellculture in a bioreactor. Generally, a fluid medium containing the cellsand the desired products are passed or flowed through a filteringdevice. The present disclosure also relates, in various otherembodiments, to a system for generating cells. In some embodiments, suchgenerated cells may be for use in a cell therapy process. In suchembodiments, the desired cells (e.g., T cells, B cells, or NK cells) arecultured and expanded in a host fluid in a bioreactor. The host fluid isthen flowed through a filtering device to capture some of the cells,while the remaining cells continue to be cultured in the bioreactor. Insome embodiments, the cells are plant cells used for bio-agriculturetechniques, for example in the production of phytochemicals or insectresistant plants.

Disclosed in various embodiments is a system comprising a bioreactor anda filtering device. The bioreactor includes a reaction vessel, anagitator, a feed inlet, and an outlet. The filtering device comprises:an inlet fluidly connected to the bioreactor outlet for receiving fluidfrom the bioreactor; a flow chamber through which the fluid can flow;and at least one ultrasonic transducer and a reflector located oppositethe at least one ultrasonic transducer, the at least one ultrasonictransducer being driven to produce a multi-dimensional standing wave inthe flow chamber.

The filtering or trapping device for the bioreactor may also consist ofa flow chamber with one or more ultrasonic transducers and reflectorsincorporated into the flow chamber. The reflectors are set up oppositethe ultrasonic transducers and the ultrasonic transducers areelectronically driven to form a multi-dimensional acoustic standing wavein the flow chamber.

The flow chamber may be made from a rigid material, such as a plastic,glass or metal container. The flow chamber may alternatively be in theform of a flexible polymeric bag or pouch that is capable of beingsealed and removed from the recirculation path between the bioreactoroutlet through the external filtering device and a recycle inlet of thebioreactor. This flexible polymeric bag or pouch may be located betweenan ultrasonic transducer and a reflector such that a multi-dimensionalacoustic standing wave may be generated interior to the flexiblepolymeric bag or pouch.

The filtering device may further comprise a product outlet through whichdesired product, such as expanded cells, viruses, exosomes, orphytochemicals are recovered. The filtering device can also furthercomprise a recycle outlet for sending fluid back to the bioreactor.

The multi-dimensional standing wave may have an axial force componentand a lateral force component which are of the same order of magnitude.The bioreactor can be operated as a perfusion bioreactor.

The sleeve may be separable from the flow chamber. Sometimes, thefiltering device further comprises a jacket located between the sleeveand the flow chamber, the jacket being used to regulate the temperatureof the fluid in the flow chamber. The jacket, the sleeve, and the flowchamber can be separable from each other and be disposable.

In particular embodiments, the ultrasonic transducer comprises apiezoelectric material that can vibrate in a higher order mode shape.The piezoelectric material may have a rectangular shape.

The ultrasonic transducer may comprise: a housing having a top end, abottom end, and an interior volume; and a crystal at the bottom end ofthe housing having an exposed exterior surface and an interior surface,the crystal being able to vibrate when driven by a voltage signal. Insome embodiments, a backing layer contacts the interior surface of thecrystal, the backing layer being made of a substantially acousticallytransparent material. The substantially acoustically transparentmaterial can be balsa wood, cork, or foam. The substantiallyacoustically transparent material may have a thickness of up to 1 inch.The substantially acoustically transparent material can be in the formof a lattice. In other embodiments, an exterior surface of the crystalis covered by a wear surface material with a thickness of a halfwavelength or less, the wear surface material being a urethane, epoxy,or silicone coating. In yet other embodiments, the crystal has nobacking layer or wear layer.

The ultrasonic transducer may also comprise a piezoelectric materialthat is polymeric such as polyvinylidene fluoride (PVDF). The PVDF maybe excited at higher frequencies up to the hundreds of megahertz rangesuch that very small particles may be trapped by the acoustic standingwave.

The multi-dimensional standing wave can be a three-dimensional standingwave.

The reflector may have a non-planar surface.

The product outlet of the filtering device may lead to a further processsuch as cell washing, cell concentration or cell fractionation. Suchprocesses may be applied when the recovered product is biological cellssuch as T cells, B cells and NK cells. In certain embodiments, the cellsused to produce viruses or exosomes are Chinese hamster ovary (CHO)cells, NS0 hybridoma cells, baby hamster kidney (BHK) cells, or humancells. The use of mammalian cell cultures including the aforementionedcell types has proven to be a very efficacious way ofproducing/expressing the recombinant proteins and monoclonal antibodiesrequired of today's pharmaceuticals. In some embodiments, the cells areplant cells that produce secondary metabolites and recombinant proteinsand other phytochemicals.

These and other non-limiting characteristics are more particularlydescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which arepresented for the purposes of illustrating the exemplary embodimentsdisclosed herein and not for the purposes of limiting the same.

FIG. 1 illustrates a single standing acoustic wave generated by anultrasonic transducer and a reflector.

FIG. 2 is an illustration comparing a fed-batch bioreactor system with aperfusion bioreactor system.

FIG. 3 is a cross-sectional view that shows the various components of abioreactor.

FIG. 4 shows one embodiment of an acoustophoretic filtering device ofthe present disclosure, with a sleeve surrounding a pipe that acts as aflow chamber and is disposable.

FIG. 5 shows another embodiment of an acoustophoretic filtering deviceof the present disclosure, showing a jacket surrounding the flowchamber, and the sleeve surrounding the jacket. The sleeve contains afluid that is used to regulate the temperature of the fluid passingthrough the flow chamber.

FIG. 6 is a schematic view illustrating a system of the presentdisclosure, including a perfusion bioreactor with an acoustophoreticseparation device, and a recycle path.

FIG. 7 is a cross-sectional diagram of a conventional ultrasonictransducer.

FIG. 8 is a picture of a wear plate of a conventional transducer.

FIG. 9 is a cross-sectional diagram of an ultrasonic transducer of thepresent disclosure. An air gap is present within the transducer, and nobacking layer or wear plate is present.

FIG. 10 is a cross-sectional diagram of an ultrasonic transducer of thepresent disclosure. An air gap is present within the transducer, and abacking layer and wear plate are present.

FIG. 11 is a graph of electrical impedance amplitude versus frequencyfor a square transducer driven at different frequencies.

FIG. 12 illustrates the trapping line configurations for seven of thepeak amplitudes of FIG. 11 from the direction orthogonal to fluid flow.

FIG. 13 is a computer simulation of the acoustic pressure amplitude(right-hand scale in Pa) and transducer out of plane displacement(left-hand scale in meters). The text at the top of the left-hand scalereads “×10⁻⁷”. The text at the top of the left-hand scale by theupward-pointing triangle reads “1.473×10⁻⁶”. The text at the bottom ofthe left-hand scale by the downward-pointing triangle reads“1.4612×10⁻¹⁰”. The text at the top of the right-hand scale reads“×10⁶”. The text at the top of the right-hand scale by theupward-pointing triangle reads “1.1129×10⁶”. The text at the bottom ofthe right-hand scale by the downward-pointing triangle reads “7.357”.The triangles show the maximum and minimum values depicted in thisfigure for the given scale. The horizontal axis is the location withinthe chamber along the X-axis, in inches, and the vertical axis is thelocation within the chamber along the Y-axis, in inches.

FIG. 14 shows the In-Plane and Out-of-Plane displacement of a crystalwhere composite waves are present.

FIG. 15 shows an exploded view of an acoustophoretic separator used forconducting some example separations, having one flow chamber.

FIG. 16 shows an exploded view of a stacked acoustophoretic separatorwith two flow chambers.

FIG. 17 is a graph showing the efficiency of removing cells from amedium using a Beckman Coulter Cell Viability Analyzer for oneexperiment.

FIG. 18 is a graph showing the efficiency of removing cells from amedium using a Beckman Coulter Cell Viability Analyzer for anotherexperiment.

FIG. 19 is an illustration of an embodiment where the filtering deviceis in the form of a flexible bag.

DETAILED DESCRIPTION

The present disclosure may be understood more readily by reference tothe following detailed description of desired embodiments and theexamples included therein. In the following specification and the claimswhich follow, reference will be made to a number of terms which shall bedefined to have the following meanings.

Although specific terms are used in the following description for thesake of clarity, these terms are intended to refer only to theparticular structure of the embodiments selected for illustration in thedrawings, and are not intended to define or limit the scope of thedisclosure. In the drawings and the following description below, it isto be understood that like numeric designations refer to components oflike function.

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

The term “comprising” is used herein as requiring the presence of thenamed component and allowing the presence of other components. The term“comprising” should be construed to include the term “consisting of”,which allows the presence of only the named component, along with anyimpurities that might result from the manufacture of the namedcomponent.

Numerical values should be understood to include numerical values whichare the same when reduced to the same number of significant figures andnumerical values which differ from the stated value by less than theexperimental error of conventional measurement technique of the typedescribed in the present application to determine the value.

All ranges disclosed herein are inclusive of the recited endpoint andindependently combinable (for example, the range of “from 2 grams to 10grams” is inclusive of the endpoints, 2 grams and 10 grams, and all theintermediate values). The endpoints of the ranges and any valuesdisclosed herein are not limited to the precise range or value; they aresufficiently imprecise to include values approximating these rangesand/or values.

The modifier “about” used in connection with a quantity is inclusive ofthe stated value and has the meaning dictated by the context (forexample, it includes at least the degree of error associated with themeasurement of the particular quantity). When used in the context of arange, the modifier “about” should also be considered as disclosing therange defined by the absolute values of the two endpoints. For example,the range of “from about 2 to about 10” also discloses the range “from 2to 10.”

It should be noted that many of the terms used herein are relativeterms. For example, the terms “upper” and “lower” are relative to eachother in location, i.e. an upper component is located at a higherelevation than a lower component in a given orientation, but these termscan change if the device is flipped. The terms “inlet” and “outlet” arerelative to a fluid flowing through them with respect to a givenstructure, e.g. a fluid flows through the inlet into the structure andflows through the outlet out of the structure. The terms “upstream” and“downstream” are relative to the direction in which a fluid flowsthrough various components, i.e. the flow fluids through an upstreamcomponent prior to flowing through the downstream component. It shouldbe noted that in a loop, a first component can be described as beingboth upstream of and downstream of a second component.

The present application refers to “the same order of magnitude.” Twonumbers are of the same order of magnitude if the quotient of the largernumber divided by the smaller number is a value of at least 1 and lessthan 10.

The term “virus” refers to an infectious agent that can only replicateinside another living cell, and otherwise exists in the form of a virionformed from a capsid that surrounds and contains DNA or RNA, and in somecases a lipid envelope surrounding the capsid.

The term “exosome” refers to a vesicle, which has a lipid bilayersurrounding a core of fluid that contains proteins, DNA, and/or RNA.

The term “crystal” refers to a single crystal or polycrystallinematerial that is used as a piezoelectric material.

Bioreactors are useful for making biomolecules such as recombinantproteins or monoclonal antibodies. Very generally, cells are cultured ina bioreactor vessel with media in order to produce the desired product,and the desired product is then harvested by separation from the cellsand media. The use of mammalian cell cultures including Chinese hamsterovary (CHO), NS0 hybridoma cells, baby hamster kidney (BHK) cells, andhuman cells has proven to be a very efficacious way ofproducing/expressing the recombinant proteins and monoclonal antibodiesused in pharmaceuticals. Two general types of bioreactor processesexist: fed-batch and perfusion.

While fed-batch reactors are the norm currently, due mainly to thefamiliarity of the process to many scientists and technicians, perfusiontechnology is growing at a very fast clip. Many factors favor the use ofa perfusion bioreactor process. The capital and start-up costs forperfusion bioreactors are lower, smaller upstream and downstreamcapacity is required, and the process uses smaller volumes and fewerseed steps than fed-batch methods. A perfusion bioreactor process alsolends itself better to development, scale-up, optimization, parametersensitivity studies, and validation.

Recent developments in perfusion bioreactor technology also favor itsuse. Control technology and general support equipment is improving forperfusion bioreactors, increasing the robustness of perfusion processes.The perfusion process can now be scaled up to bioreactors having avolume up to 1000 liters (L). Better cell retention systems forperfusion bioreactors result in lower cell loss and greater celldensities than have been seen previously. Cell densities greater than 50million cells/mL are now achievable, compared to fed-batch celldensities of around 20 million cells/mL. Lower contamination andinfection rates have improved the output of perfusion bioreactors.Higher product concentrations in the harvest and better yields withoutsignificant increase in cost have thus resulted for perfusion processes.

A separate aspect of the use of high cell concentration bioreactors isthe “dewatering” of the materials at the end of a bioreactor run. The“dewatering” or removal of interstitial fluid from a bioreactor sludgeis important for improving the efficiency of recovery of the intendedbioreactor product. Currently, high energy centrifuges with internalstructures (known as disk stack centrifuges) are utilized to remove theinterstitial fluid from the bioreactor sludge at the end of a run. Thecapital cost and operating costs for a disk stack centrifuge is high. Asimpler method of removing the interstitial fluid from the remainingbioreactor sludge that can be performed without the high capital andoperating costs associated with disk stack centrifuges is desirable. Inaddition, current methods of filtration or centrifugation can damagecells, releasing protein debris and enzymes into the purificationprocess and increasing the load on downstream portions of thepurification system.

Briefly, the present disclosure relates to the generation ofmulti-dimensional (e.g., three-dimensional (3-D)) acoustic standingwave(s) from one or more piezoelectric transducers, where thetransducers are electrically or mechanically excited such that they movein a “drumhead” or multi-excitation mode (i.e., multi-mode displacementpattern), rather than a “piston” or single excitation mode fashion.Through this manner of acoustic standing wave generation, a higherlateral trapping force is generated than if the piezoelectric transduceris excited in a “piston” mode where only one large standing wave isgenerated. Thus, with the same input power to a piezoelectrictransducer, the multi-dimensional (e.g., 3-D) acoustic standing wave(s)can have a higher lateral trapping force compared to a single planaracoustic standing wave. The input power is tunable for a controlledflow. This can be used to facilitate proteinaceous fluid purification ofthe contents of a bioreactor. Thus, the present disclosure relates toprocessing systems comprising a bioreactor and a filtering device, thefiltering device using acoustophoresis for separation of variouscomponents.

Through utilization of an acoustophoretic filtering device thatincorporates a multi-dimensional (e.g., 3-D) standing wave, maintainingflux rates and minimizing cross-contamination risk in a multiproductsystem can also be achieved. Other benefits, such as cleaning proceduresand related demands often detailed and validated within standardoperating procedures (SOP), can also be realized through the use of amulti-dimensional (e.g., 3-D) acoustic standing wave capable apparatus.The cross-contamination risk can be eliminated between the bioreactorand outside processes.

Acoustophoresis is a low-power, no-pressure-drop, no-clog, solid-stateapproach to particle removal from fluid dispersions: i.e., it is used toachieve separations that are more typically performed with porousfilters, but it has none of the disadvantages of filters. In particular,the present disclosure provides filtering devices that are suitable foruse with bioreactors and operate at the macro-scale for separations inflowing systems with high flow rates. The acoustophoretic filteringdevice is designed to create a high intensity multi-dimensional (e.g.,three dimensional) ultrasonic standing wave that results in an acousticradiation force that is larger than and can overcome the combinedeffects of fluid drag and buoyancy or gravity at certain flow rates, andis therefore able to trap (i.e., hold stationary) the suspended phase(i.e. cells) to allow more time for the acoustic wave to increaseparticle concentration, agglomeration and/or coalescence. Put anotherway, the radiation force of the acoustic standing wave(s) acts as afilter that prevents or retards targeted particles (e.g., biologicalcells) from crossing through the standing wave(s). The present systemshave the ability to create ultrasonic standing wave fields that can trapparticles in flow fields with a linear velocity ranging from 0.1 mm/secto velocities exceeding 1 cm/s. As explained above, the trappingcapability of a standing wave may be varied as desired, for example byvarying the flow rate of the fluid, the acoustic radiation force, andthe shape of the acoustic filtering device to maximize cell retentionthrough trapping and settling. This technology offers a green andsustainable alternative for separation of secondary phases with asignificant reduction in cost of energy. Excellent particle separationefficiencies have been demonstrated for particle sizes as small as onemicron.

The ultrasonic standing waves can be used to trap, i.e., holdstationary, secondary phase particles (e.g. cells) in a host fluidstream (e.g. cell culture media). This is an important distinction fromprevious approaches where particle trajectories were merely altered bythe effect of the acoustic radiation force. The scattering of theacoustic field off the particles results in a three dimensional acousticradiation force, which acts as a three-dimensional trapping field. Theacoustic radiation force is proportional to the particle volume (e.g.the cube of the radius) when the particle is small relative to thewavelength. It is proportional to frequency and the acoustic contrastfactor. It also scales with acoustic energy (e.g. the square of theacoustic pressure amplitude). For harmonic excitation, the sinusoidalspatial variation of the force is what drives the particles to thestable positions within the standing waves. When the acoustic radiationforce exerted on the particles is stronger than the combined effect offluid drag force and buoyancy/gravitational force, the particle istrapped within the acoustic standing wave field. The action of theacoustic forces (i.e., the lateral and axial acoustic forces) on thetrapped particles results in formation of tightly-packed clustersthrough concentration, clustering, clumping, agglomeration and/orcoalescence of particles that, when reaching a critical size, settlecontinuously through enhanced gravity for particles heavier than thehost fluid or rise out through enhanced buoyancy for particles lighterthan the host fluid. Additionally, secondary inter-particle forces, suchas Bjerkness forces, aid in particle agglomeration.

Generally, the 3-D standing wave(s) filtering system is operated at avoltage such that the protein-producing materials, such as Chinesehamster ovary cells (CHO cells), the most common host for the industrialproduction of recombinant protein therapeutics, are trapped in theultrasonic standing wave, i.e., remain in a stationary position. Withineach nodal plane, the CHO cells are trapped in the minima of theacoustic radiation potential. Most biological cell types present ahigher density and lower compressibility than the medium in which theyare suspended, so that the acoustic contrast factor between the cellsand the medium has a positive value. As a result, the axial acousticradiation force (ARF) drives the biological cells towards the standingwave pressure nodes. The axial component of the acoustic radiation forcedrives the cells, with a positive contrast factor, to the pressure nodalplanes, whereas cells or other particles with a negative contrast factorare driven to the pressure anti-nodal planes. The radial or lateralcomponent of the acoustic radiation force is the force that traps thecells. The radial or lateral component of the ARF is larger than thecombined effect of fluid drag force and gravitational force. For smallcells or emulsions the drag force F_(D) can be expressed as:

${{\overset{\rightarrow}{F}}_{D} = {4\; \pi \; \mu_{f}{{R_{p}\left( {{\overset{\rightarrow}{U}}_{f} - {\overset{\rightarrow}{U}}_{p}} \right)}\left\lbrack \frac{1 + {\frac{3}{2}\hat{\mu}}}{1 + \hat{\mu}} \right\rbrack}}},$

where U_(f) and U_(p) are the fluid and cell velocity, R_(p) is theparticle radius, μ_(f) and μ_(p) are the dynamic viscosity of the fluidand the cells, and {circumflex over (μ)}=μ_(p)/μ_(f) is the ratio ofdynamic viscosities. The buoyancy force F_(B) is expressed as:

$F_{B} = {\frac{4}{3}\pi \; {{R_{p}^{3}\left( {\rho_{f} - \rho_{p}} \right)}.}}$

To determine when a cell is trapped in the ultrasonic standing wave, theforce balance on the cell may be assumed to be zero, and therefore anexpression for lateral acoustic radiation force F_(LRF) can be found,which is given by:

F _(LRF) =F _(D) +F _(B).

For a cell of known size and material property, and for a given flowrate, this equation can be used to estimate the magnitude of the lateralacoustic radiation force.

One theoretical model that is used to calculate the acoustic radiationforce is based on the formulation developed by Gor'kov. The primaryacoustic radiation force F_(A) is defined as a function of a fieldpotential U, F_(A)=−∇(U),

where the field potential U is defined as

${U = {V_{0}\left\lbrack {{\frac{\langle p^{2}\rangle}{2\; \rho_{f}c_{f}^{2}}f_{1}} - {\frac{3\; \rho_{f}{\langle u^{2}\rangle}}{4}f_{2}}} \right\rbrack}},$

and f₁ and f₂ are the monopole and dipole contributions defined by

${f_{1} = {1 - \frac{1}{\Lambda \; \sigma^{2}}}},{f_{2} = \frac{2\left( {\Lambda - 1} \right)}{{2\; \Lambda} + 1}},$

where p is the acoustic pressure, u is the fluid particle velocity, Λ isthe ratio of cell density ρ_(p) to fluid density ρ_(f), σ is the ratioof cell sound speed c_(p) to fluid sound speed c_(f), V_(o) is thevolume of the cell, and < > indicates time averaging over the period ofthe wave.

Gor'kov's theory is limited to particle sizes that are small withrespect to the wavelength of the sound fields in the fluid and theparticle, and it also does not take into account the effect of viscosityof the fluid and the particle on the radiation force. Additionaltheoretical and numerical models have been developed for the calculationof the acoustic radiation force for a particle without any restrictionas to particle size relative to wavelength. These models also includethe effect of fluid and particle viscosity, and therefore are a moreaccurate calculation of the acoustic radiation force. The models thatwere implemented are based on the theoretical work of Yurii Ilinskii andEvgenia Zabolotskaya as described in AIP Conference Proceedings, Vol.1474-1, pp. 255-258 (2012). Additional in-house models have beendeveloped to calculate acoustic trapping forces for cylindrical shapedobjects, such as the “hockey pucks” of trapped particles in the standingwave, which closely resemble a cylinder.

The lateral force component of the total acoustic radiation force (ARF)generated by the ultrasonic transducer(s) of the present disclosure issignificant and is sufficient to overcome the fluid drag force at linearvelocities of up to 1 cm/s, and to create tightly packed clusters, andis of the same order of magnitude as the axial force component of thetotal acoustic radiation force. This lateral ARF can thus be used toretain cells in a bioreactor while the bioreactor process continues.This is especially true for a perfusion bioreactor.

The filtering devices of the present disclosure, which use ultrasonictransducers and acoustophoresis, can also improve the dewatering of theleftover material from a bioreactor batch (i.e bioreactor sludge), andthus reduce the use of or eliminate the use of disk stack centrifuges.This use or application of ultrasonic transducers and acoustophoresissimplifies processing and reduces costs.

In a perfusion bioreactor system, it is desirable to be able to filterand separate the cells and cell debris from the expressed materials thatare in the fluid stream (i.e. cell culture media). The expressedmaterials are composed of biomolecules such as recombinant proteins ormonoclonal antibodies, and are the desired product to be recovered.

An acoustophoretic filtering device can be used in at least twodifferent ways. First, the standing waves can be used to trap theexpressed biomolecules and separate this desired product from the cells,cell debris, and media. The expressed biomolecules can then be divertedand collected for further processing. Alternatively, the standing wavescan be used to trap the cells and cell debris present in the cellculture media. The cells and cell debris, having a positive contrastfactor, move to the nodes (as opposed to the anti-nodes) of the standingwave. As the cells and cell debris agglomerate at the nodes of thestanding wave, there is also a physical scrubbing of the cell culturemedia that occurs whereby more cells are trapped as they come in contactwith the cells that are already held within the standing wave. Thisgenerally separates the cells and cellular debris from the cell culturemedia. When the cells in the standing wave agglomerate to the extentwhere the mass is no longer able to be held by the acoustic wave, theaggregated cells and cellular debris that have been trapped can fall outof the fluid stream through gravity, and can be collected separately. Toaid this gravitational settling of the cells and cell debris, thestanding wave may be interrupted to allow all of the cells to fall outof the fluid stream that is being filtered. This process can be usefulfor dewatering. The expressed biomolecules may have been removedbeforehand, or remain in the fluid stream (i.e. cell culture medium).

Desirably, the ultrasonic transducer(s) generate a multi-dimensional(e.g., three-dimensional) standing wave in the fluid that exerts alateral force on the suspended particles to accompany the axial force soas to increase the particle trapping capabilities of the acoustophoreticfiltering device. Typical results published in literature state that thelateral force is two orders of magnitude smaller than the axial force.In contrast, the technology disclosed in this application provides for alateral force to be of the same order of magnitude as the axial force.

In the present disclosure, a perfusion bioreactor can also be used togenerate cells that can subsequently be used for cell therapy. In thistype of process, the biological cells to be used in the cell therapy arecultured in the bioreactor and expanded (i.e. to increase the number ofcells in the bioreactor through cell reproduction). These cells may belymphocytes such as T cells (e.g., regulatory T-cells (Tregs), JurkatT-cells), B cells, or NK cells; their precursors, such as peripheralblood mononuclear cells (PBMCs); and the like. The cell culture media(aka host fluid), containing some cells, is then sent to a filteringdevice that produces an acoustic standing wave. A portion of the cellsare trapped and held in the acoustic standing wave, while the remaininghost fluid and other cells in the host fluid are returned to thebioreactor. As the quantity of trapped cells increases, they form largerclusters that will fall out of the acoustic standing wave at a criticalsize due to gravity forces. The clusters can fall into a product outletoutside a region of the acoustic standing wave, such as below theacoustic standing wave, from which the cells can be recovered for use incell therapy. Only a small portion of the cells are trapped and removedfrom the bioreactor via the product outlet, and the remainder continueto reproduce in the bioreactor, allowing for continuous production andrecovery of the desired cells.

In another application, acoustic standing waves are used to trap andhold biological cells and to separate viruses (e.g. lentiviruses) orexosomes that are produced by the biological cells. In theseembodiments, the biological cells are returned to the bioreactorpost-separation to continue production of viruses or exosomes.

In these applications, the acoustic filtering devices of the presentdisclosure can act as a cell retention device. The acoustic cellretention systems described herein operate over a range of cellrecirculation rates, efficiently retain cells over a range of perfusion(or media removal) rates, and can be tuned to fully retain orselectively pass some percentage of cells through fluid flow rate,transducer power or frequency manipulation. Power and flow rates can allbe monitored and used as feedback in an automated control system.

The cells of interest may also be held in the flow chamber of theexternal filtering device through the use of an acoustic standing wavesuch that other moieties may be introduced in close proximity to and forthe purpose of changing the target cells. Such an operation wouldinclude the trapping of T cells and the subsequent introduction ofmodified lentivirus materials with a specific gene splice such that thelentivirus with a specific gene splice will transfect the T cell andgenerate a chimeric antigen receptor T cell also known as a CAR-T cell.

The acoustic filtering devices of the present disclosure are designed tomaintain a high intensity three-dimensional acoustic standing wave. Thedevice is driven by a function generator and amplifier (not shown). Thedevice performance is monitored and controlled by a computer. It may bedesirable, at times, due to acoustic streaming, to modulate thefrequency or voltage amplitude of the standing wave. This modulation maybe done by amplitude modulation and/or by frequency modulation. The dutycycle of the propagation of the standing wave may also be utilized toachieve certain results for trapping of materials. In other words, theacoustic beam may be turned on and shut off at different frequencies toachieve desired results.

FIG. 1 illustrates a single standing wave system 100 that is comprisedof a reflector plate 101 and an ultrasonic transducer 103 that is set toresonate so as to form a standing wave 102. Excitation frequenciestypically in the range from hundreds of kHz to tens of MHz are appliedby the transducer 103. One or more standing waves are created betweenthe transducer 103 and the reflector 101. The standing wave is the sumof two propagating waves that are equal in frequency and intensity andthat are traveling in opposite directions, i.e. from the transducer tothe reflector and back. The propagating waves destructively interferewith each other and thus generate the standing wave. A dotted line 105is used to indicate the amplitude. A node is a point where the wave hasminimum amplitude, and is indicated with reference numeral 107. Ananti-node is a point where the wave has maximum amplitude, and isindicated with reference numeral 109.

FIG. 2 is a schematic diagram that compares a fed-batch bioreactorsystem 201 (left side) with a perfusion bioreactor system 202 (rightside). Beginning with the fed-batch bioreactor on the left, thebioreactor 210 includes a reaction vessel 220. The cell culture media isfed to the reaction vessel through a feed inlet 222. An agitator 225 isused to circulate the media throughout the cell culture. Here, theagitator is depicted as a set of rotating blades, though any type ofsystem that causes circulation is contemplated. The bioreactor permitsgrowth of a seed culture through a growth/production cycle, during whichtime debris, waste and unusable cells will accumulate in the bioreactorand the desired product (e.g. biomolecules such as monoclonalantibodies, recombinant proteins, hormones, etc.) will be produced aswell. Due to this accumulation, the reaction vessel of a fed-batchprocess is typically much larger than that in a perfusion process. Thedesired product is then harvested at the end of the production cycle.The reaction vessel 220 also includes an outlet 224 for removingmaterial.

Turning now to the perfusion bioreactor 202 on the right-hand side,again, the bioreactor includes a reaction vessel 220 with a feed inlet222 for the cell culture media. An agitator 225 is used to circulate themedia throughout the cell culture. An outlet 224 of the reaction vesselis fluidly connected to the inlet 232 of a filtering device 230, andcontinuously feeds the media (containing cells and desired product) tothe filtering device. The filtering device 230 is located downstream ofthe reaction vessel, and separates the desired product from the cells.The filtering device 230 has two separate outlets, a product outlet 234and a recycle outlet 236. The product outlet 234 fluidly connects thefiltering device 230 to a containment vessel 240 downstream of thefiltering device, which receives a concentrated flow of the desiredproduct (plus media) from the filtering device. From there, furtherprocessing/purification can occur to isolate/recover the desiredproduct. The recycle outlet 236 fluidly connects the filtering device230 back to a recycle inlet 226 of the reaction vessel 220, and is usedto send the cells and cell culture media back into the reaction vesselfor continued growth/production. Put another way, there is a fluid loopbetween the reaction vessel and the filtering device. The reactionvessel 220 in the perfusion bioreactor system 202 has a continuousthroughput of product and thus can be made smaller than the fed-batchbioreactor system 201. The filtering process is critical to thethroughput of the perfusion bioreactor. A poor filtering process willallow for only low throughput and result in low yields of the desiredproduct.

FIG. 3 is a cross-sectional view of a generic bioreactor 300 that isuseful for the systems of the present disclosure. As illustrated here,the bioreactor includes a reaction vessel 320 having an internal volume323. A feed inlet 322 at the top of the vessel is used to feed cellculture media into the vessel. An agitator 325 is present. An outlet 324is shown at the bottom of the vessel. A thermal jacket 310 surrounds thereaction vessel, and is used to regulate the temperature of thecells/media. An aerator 312 is located on the bottom of the vessel forproviding gas to the internal volume. Sensors 314 are shown at the topright of the vessel. A pump 316 is illustrated for feeding the cellculture media into the vessel, as is another pump 318 for removing cellculture media from the vessel. An interior light source for illuminatingthe internal volume may be present, for example when the bioreactor isused for growing plant cells.

The perfusion systems of the present disclosure also use anacoustophoretic filtering device. The contents of the bioreactor arecontinuously flowed through the filtering device to capture the desiredproducts.

FIG. 4 is a first embodiment of an acoustophoretic filtering device 400.The device includes a flow chamber 410, which is depicted here as acylindrical pipe or tube. A feed inlet 412 is illustrated here at thebottom of the flow chamber, through which fluid from the bioreactor isreceived. An outlet 414 is depicted at the top of the flow chamber, withthe arrows (reference numeral 415) indicating the direction of fluidflow. A sleeve 420 surrounds the flow chamber. The sleeve includes atleast one ultrasonic transducer 422 and at least one reflector 424,which are located opposite each other. Together, the transducer andreflector generate one or more standing waves 425, with the reflectorbouncing the initial propagated wave back towards the transducer with asimilar frequency and intensity to form an acoustic standing wave. It isparticularly contemplated that the sleeve can be separated from the flowchamber/pipe. The pipe can be discarded and replaced with a new pipe.This configuration allows for disposable parts in the filtering device,and thus reduces the cost of cleaning and sterilization that mightotherwise be incurred with a permanent filter. It is noted that thefiltering device may include additional inlets or outlets not depictedhere, as previously explained.

FIG. 5 is a second embodiment of the acoustophoretic filtering device.Here, the filtering device 400 also includes a jacket 430 that islocated between the sleeve 420 and the flow chamber 410. The jacketcontains a temperature-regulating fluid 432 that can be used to controlthe temperature of the fluid passing through the flow chamber. In thisregard, it is usually desirable to maintain the temperature of the cellculture below 38° C. to prevent compromise of the cells. Thetemperature-regulating fluid is completely separated from the cellculture media/fluid passing through the flow chamber 410. It is notedthat the standing wave 425 created by the transducer 422 and reflector424 will propagate through the jacket 430 and the temperature regulatingfluid 432 therein, and will continue to operate in the flow chamber toseparate the targeted material in the flow chamber.

FIG. 6 illustrates an exemplary processing system of the presentdisclosure, comprising a bioreactor 610 and a filtering device 630. Thesystem is set up for use as a perfusion bioreactor. The bioreactor 610includes a reaction vessel 620 having a feed inlet 622, an outlet 624,and a recycle inlet 626. Media is added into the feed inlet 622 by anaddition pipe 650. The contents of the reaction vessel (referencenumeral 605) are mixed with an agitator 625. The desired product (e.g.recombinant proteins, viruses, exosomes, or additional cells) iscontinuously produced by the cells located within the vessel 620, andare present in the media of the bioreactor. The product and the cells inthe perfusion bioreactor are drawn from the reaction vessel through pipe652, and enter the acoustophoretic filtering device 630 through inlet632. There, the desired product is separated from the cells through theuse of multi-dimensional standing waves. The desired product can bedrawn off through a product outlet 634 and pipe 654 into a containmentvessel 640. The cells are returned to the perfusion bioreactor afterseparation, passing from recycle outlet 636 of the filtering devicethrough pipe 656 to recycle inlet 626 of the reaction vessel, which forma recycle path. The 3-D standing waves of the acoustophoresis deviceallow for high throughput of the perfusion reactor due to the increasedlateral trapping force of the 3-D standing waves. It is noted thatalthough the reaction vessel outlet 624 is depicted at the top of thevessel and the recycle inlet 626 is depicted at the bottom of thevessel, that this arrangement can be reversed if desired. This maydepend on the desired product to be obtained.

In additional embodiments, it is particularly contemplated that thefiltering device 630 is in the form of a flexible bag or pouch. Such afiltering device is illustrated in FIG. 19. The interior volume of theflexible bag 700 operates as the flow chamber. The flexible bag includesan inlet 702 and an outlet 704. Opposite surfaces of the flexible bagcan be stiff. One surface includes an ultrasonic transducer 710, and theopposite surface includes a reflector 712 opposite the transducer, sothat a multi-dimensional acoustic standing wave can be generated withinthe bag.

Referring to both FIG. 6 and FIG. 7, cell culture media and cells aredrawn from the reaction vessel through pipe 652, and enter the flexiblebag 700 through inlet 702. The multi-dimensional acoustic standing wavetraps the desired product (i.e. cells). The cell culture media and othermaterial exit through the outlet 704 through pipe 656 back to recycleinlet 626 of the reaction vessel. Eventually, as the bag fills up withconcentrated cells, the fluid flow through the bag 700 is stopped. Thebag filled with concentrated cells can then be taken out of the recyclepath between the product outlet 634 and the recycle inlet 626 of thereaction vessel. The concentrated cells are then recovered from the bag.

In other embodiments, it is particularly contemplated that a flexiblebag or pouch is used within the flow chamber of the filtering device 630for the capture of cells. This flexible bag or pouch is similar to thebag 700 of FIG. 19, but does not have the ultrasonic transducer andreflector attached thereto. Cell culture media and cells are drawn fromthe reaction vessel through pipe 652, and enter the acoustophoreticfiltering device 630 through inlet 632. The flexible bag itself containsan inlet and an outlet, and the acoustophoretic filtering device acts asa housing for the bag. The multi-dimensional acoustic standing wavetraps the desired product (i.e. cells). The cell culture media and othermaterial exit through the outlet of the bag and out through recycleoutlet 636 of the filtering device through pipe 656 to recycle inlet 626of the reaction vessel, which form a recycle path.

Within the flexible bag, as the quantity of trapped cells increases, thecells form larger clusters that will fall out of the acoustic standingwave at a critical size due to gravity forces. The clusters fall to thebottom of the bag. Eventually, as the bag fills up with concentratedcells, the fluid flow through the filtering device 630 is stopped. Thebag filled with concentrated cells can then be removed and a new bagplaced within the filtering device 630. Referring to FIG. 6, thefiltering device inlet 632 would probably be near the middle of thefiltering device 630, and the recycle outlet 636 would probably belocated at the top of the filtering device, with the concentrated cellsfalling to the bottom of the flexible bag to be collected. No productoutlet 634 or containment vessel 640 would be needed to collect theproduct, which would be collected in the flexible bag that issubsequently removed from the flow chamber of the filtering device 630.

It may be helpful now to describe the ultrasonic transducer(s) used inthe acoustophoretic filtering device in more detail. FIG. 7 is across-sectional diagram of a conventional ultrasonic transducer. Thistransducer has a wear plate 50 at a bottom end, epoxy layer 52, ceramiccrystal 54 (made of, e.g. Lead Zirconate Titanate (PZT)), an epoxy layer56, and a backing layer 58. On either side of the ceramic crystal, thereis an electrode: a positive electrode 61 and a negative electrode 63.The epoxy layer 56 attaches backing layer 58 to the crystal 54. Theentire assembly is contained in a housing 60 which may be made out of,for example, aluminum. An electrical adapter 62 provides connection forwires to pass through the housing and connect to leads (not shown) whichattach to the crystal 54. Typically, backing layers are designed to adddamping and to create a broadband transducer with uniform displacementacross a wide range of frequency and are designed to suppress excitationat particular vibrational eigen-modes. Wear plates are usually designedas impedance transformers to better match the characteristic impedanceof the medium into which the transducer radiates.

FIG. 8 is a photo of a wear plate 50 with a bubble 64 where the wearplate has pulled away from the ceramic crystal surface due to theoscillating pressure and heating.

FIG. 9 is a cross-sectional view of an ultrasonic transducer 81 of thepresent disclosure, which is used in the acoustophoretic filteringdevices of the present disclosure. Transducer 81 has an aluminum housing82. A PZT crystal 86 defines the bottom end of the transducer, and isexposed from the exterior of the housing. The crystal is supported onits perimeter by a small elastic layer 98, e.g. silicone or similarmaterial, located between the crystal and the housing. Put another way,no wear layer is present.

Screws (not shown) attach an aluminum top plate 82 a of the housing tothe body 82 b of the housing via threads 88. The top plate includes aconnector 84 to pass power to the PZT crystal 86. The bottom and topsurfaces of the PZT crystal 86 are each connected to an electrode(positive and negative), such as silver or nickel. A wrap-aroundelectrode tab 90 connects to the bottom electrode and is isolated fromthe top electrode. Electrical power is provided to the PZT crystal 86through the electrodes on the crystal, with the wrap-around tab 90 beingthe ground connection point. Note that the crystal 86 has no backinglayer or epoxy layer as is present in FIG. 5. Put another way, there isan air gap 87 in the transducer between aluminum top plate 82 a and thecrystal 86 (i.e. the air gap is completely empty). A minimal backing 58and/or wear plate 50 may be provided in some embodiments, as seen inFIG. 10.

The transducer design can affect performance of the system. A typicaltransducer is a layered structure with the ceramic crystal bonded to abacking layer and a wear plate. Because the transducer is loaded withthe high mechanical impedance presented by the standing wave, thetraditional design guidelines for wear plates, e.g., half wavelengththickness for standing wave applications or quarter wavelength thicknessfor radiation applications, and manufacturing methods may not beappropriate. Rather, in one embodiment of the present disclosure thetransducers, there is no wear plate or backing, allowing the crystal tovibrate in one of its eigenmodes with a high Q-factor. The vibratingceramic crystal/disk is directly exposed to the fluid flowing throughthe flow chamber.

Removing the backing (e.g. making the crystal air backed) also permitsthe ceramic crystal to vibrate at higher order modes of vibration withlittle damping (e.g. higher order modal displacement). In a transducerhaving a crystal with a backing, the crystal vibrates with a moreuniform displacement, like a piston. Removing the backing allows thecrystal to vibrate in a non-uniform displacement mode. The higher orderthe mode shape of the crystal, the more nodal lines the crystal has. Thehigher order modal displacement of the crystal creates more trappinglines, although the correlation of trapping line to node is notnecessarily one to one, and driving the crystal at a higher frequencywill not necessarily produce more trapping lines.

In some embodiments, the crystal may have a backing that minimallyaffects the Q-factor of the crystal (e.g. less than 5%). The backing maybe made of a substantially acoustically transparent material such asbalsa wood, foam, or cork which allows the crystal to vibrate in ahigher order mode shape and maintains a high Q-factor while stillproviding some mechanical support for the crystal. The backing layer maybe a solid, or may be a lattice having holes through the layer, suchthat the lattice follows the nodes of the vibrating crystal in aparticular higher order vibration mode, providing support at nodelocations while allowing the rest of the crystal to vibrate freely. Thegoal of the lattice work or acoustically transparent material is toprovide support without lowering the Q-factor of the crystal orinterfering with the excitation of a particular mode shape.

Placing the crystal in direct contact with the fluid also contributes tothe high Q-factor by avoiding the dampening and energy absorptioneffects of the epoxy layer and the wear plate. Other embodiments mayhave wear plates or a wear surface to prevent the PZT, which containslead, contacting the host fluid. This may be desirable in, for example,biological applications such as separating blood. Such applicationsmight use a wear layer such as chrome, electrolytic nickel, orelectroless nickel. Chemical vapor deposition could also be used toapply a layer of poly(p-xylylene) (e.g. Parylene) or other polymer.Organic and biocompatible coatings such as silicone or polyurethane arealso usable as a wear surface.

In some embodiments, the ultrasonic transducer has a 1 inch diameter anda nominal 2 MHz resonance frequency. Each transducer can consume about28 W of power for droplet trapping at a flow rate of 3 GPM. This powerusage translates to an energy cost of 0.25 kW hr/m³. This measure is anindication of the very low cost of energy of this technology. Desirably,each transducer is powered and controlled by its own amplifier. In otherembodiments, the ultrasonic transducer uses a square crystal, forexample with 1″×1″ dimensions. Alternatively, the ultrasonic transducercan use a rectangular crystal, for example with 1″×2.5″ dimensions.Power dissipation per transducer was 10 W per 1″×1″ transducercross-sectional area and per inch of acoustic standing wave span inorder to get sufficient acoustic trapping forces. For a 4″ span of anintermediate scale system, each 1″×1″ square transducer consumes 40 W.The larger 1″×2.5″ rectangular transducer uses 100 W in an intermediatescale system. The array of three 1″×1″ square transducers would consumea total of 120 W and the array of two 1″×2.5″ transducers would consumeabout 200 W. Arrays of closely spaced transducers represent alternatepotential embodiments of the technology. Transducer size, shape, number,and location can be varied as desired to generate desiredthree-dimensional acoustic standing wave patterns.

The size, shape, and thickness of the transducer determine thetransducer displacement at different frequencies of excitation, which inturn affects separation efficiency. Typically, the transducer isoperated at frequencies near the thickness resonance frequency (halfwavelength). Gradients in transducer displacement typically result inmore trapping locations for the cells/biomolecules. Higher order modaldisplacements generate three-dimensional acoustic standing waves withstrong gradients in the acoustic field in all directions, therebycreating equally strong acoustic radiation forces in all directions,leading to multiple trapping lines, where the number of trapping linescorrelate with the particular mode shape of the transducer.

To investigate the effect of the transducer displacement profile onacoustic trapping force and separation efficiencies, an experiment wasrepeated ten times using a 1″×1″ square transducer, with all conditionsidentical except for the excitation frequency. Ten consecutive acousticresonance frequencies, indicated by circled numbers 1-9 and letter A onFIG. 11, were used as excitation frequencies. The conditions wereexperiment duration of 30 min, a 1000 ppm oil concentration ofapproximately 5-micron SAE-30 oil droplets, a flow rate of 500 ml/min,and an applied power of 20 W. Oil droplets were used because oil isdenser than water, and can be separated from water usingacoustophoresis.

FIG. 11 shows the measured electrical impedance amplitude of a squaretransducer as a function of frequency in the vicinity of the 2.2 MHztransducer resonance. The minima in the transducer electrical impedancecorrespond to acoustic resonances of the water column and representpotential frequencies for operation. Numerical modeling has indicatedthat the transducer displacement profile varies significantly at theseacoustic resonance frequencies, and thereby directly affects theacoustic standing wave and resulting trapping force. Since thetransducer operates near its thickness resonance, the displacements ofthe electrode surfaces are essentially out of phase. The typicaldisplacement of the transducer electrodes is not uniform and variesdepending on frequency of excitation. As an example, at one frequency ofexcitation with a single line of trapped oil droplets, the displacementhas a single maximum in the middle of the electrode and minima near thetransducer edges. At another excitation frequency, the transducerprofile has multiple maxima leading to multiple trapped lines of oildroplets. Higher order transducer displacement patterns result in highertrapping forces and multiple stable trapping lines for the captured oildroplets.

As the oil-water emulsion passed by the transducer, the trapping linesof oil droplets were observed and characterized. The characterizationinvolved the observation and pattern of the number of trapping linesacross the fluid channel, as shown in FIG. 12, for seven of the tenresonance frequencies identified in FIG. 11. Different displacementprofiles of the transducer can produce different (more) trapping linesin the standing waves, with more gradients in displacement profilegenerally creating higher trapping forces and more trapping lines.

FIG. 13 is a numerical model showing a pressure field that matches the 9trapping line pattern. The numerical model is a two-dimensional model;and therefore only three trapping lines are observed. Two more sets ofthree trapping lines exist in the third dimension perpendicular to theplane of the page.

In the present system examples, the system is operated at a voltage suchthat the particles (i.e. biomolecules or cells) are trapped in theultrasonic standing wave, i.e., remain in a stationary position. Theparticles are collected in along well defined trapping lines, separatedby half a wavelength. Within each nodal plane, the particles are trappedin the minima of the acoustic radiation potential. The axial componentof the acoustic radiation force drives particles with a positivecontrast factor to the pressure nodal planes, whereas particles with anegative contrast factor are driven to the pressure anti-nodal planes.The radial or lateral component of the acoustic radiation force is theforce that traps the particle. It therefore, for particle trapping, islarger than the combined effect of fluid drag force and gravitationalforce. In systems using typical transducers, the radial or lateralcomponent of the acoustic radiation force is typically several orders ofmagnitude smaller than the axial component of the acoustic radiationforce. However, the lateral force generated by the transducers of thepresent disclosure can be significant, on the same order of magnitude asthe axial force component, and is sufficient to overcome the fluid dragforce at linear velocities of up to 1 cm/s.

The lateral force can be increased by driving the transducer in higherorder mode shapes, as opposed to a form of vibration where the crystaleffectively moves as a piston having a uniform displacement. Theacoustic pressure is proportional to the driving voltage of thetransducer. The electrical power is proportional to the square of thevoltage. The transducer is typically a thin piezoelectric plate, withelectric field in the z-axis and primary displacement in the z-axis. Thetransducer is typically coupled on one side by air (i.e. the air gapwithin the transducer) and on the other side by the fluid of the cellculture media. The types of waves generated in the plate are known ascomposite waves. A subset of composite waves in the piezoelectric plateis similar to leaky symmetric (also referred to as compressional orextensional) Lamb waves. The piezoelectric nature of the plate typicallyresults in the excitation of symmetric Lamb waves. The waves are leakybecause they radiate into the water layer, which result in thegeneration of the acoustic standing waves in the water layer. Lamb wavesexist in thin plates of infinite extent with stress free conditions onits surfaces. Because the transducers of this embodiment are finite innature, the actual modal displacements are more complicated.

FIG. 14 shows the typical variation of the in-plane displacement(x-displacement) and out-of-plane displacement (y-displacement) acrossthe thickness of the plate, the in-plane displacement being an evenfunction across the thickness of the plate and the out-of-planedisplacement being an odd function. Because of the finite size of theplate, the displacement components vary across the width and length ofthe plate. In general, a (m, n) mode is a displacement mode of thetransducer in which there are m undulations in transducer displacementin the width direction and n undulations in the length direction, andwith the thickness variation as described in FIG. 14. The maximum numberof m and n is a function of the dimension of the crystal and thefrequency of excitation.

The transducers are driven so that the piezoelectric crystal vibrates inhigher order modes of the general formula (m, n), where m and n areindependently 1 or greater. Generally, the transducers will vibrate inhigher order modes than (2,2). Higher order modes will produce morenodes and antinodes, result in three-dimensional standing waves in thefluid layer, characterized by strong gradients in the acoustic field inall directions, not only in the direction of the standing waves, butalso in the lateral directions. As a consequence, the acoustic gradientsresult in stronger trapping forces in the lateral direction.

In embodiments, the pulsed voltage signal driving the transducer canhave a sinusoidal, square, sawtooth, or triangle waveform; and have afrequency of 500 kHz to 10 MHz. The pulsed voltage signal can be drivenwith pulse width modulation, which produces any desired waveform. Thepulsed voltage signal can also have amplitude or frequency modulationstart/stop capability to eliminate streaming.

The transducer(s) is/are used to create a pressure field that generatesforces of the same order of magnitude both orthogonal to the standingwave direction and in the standing wave direction. When the forces areroughly the same order of magnitude, particles of size 0.1 microns to300 microns will be moved more effectively towards regions ofagglomeration (“trapping lines”). Because of the equally large gradientsin the orthogonal acoustophoretic force component, there are “hot spots”or particle collection regions that are not located in the regularlocations in the standing wave direction between the transducer and thereflector. Hot spots are located in the maxima or minima of acousticradiation potential. Such hot spots represent particle collectionlocations which allow for better wave transmission between thetransducer and the reflector during collection and strongerinter-particle forces, leading to faster and better particleagglomeration.

FIG. 15 and FIG. 16 are exploded views showing the various parts ofacoustophoretic separators. FIG. 15 has only one separation chamber,while FIG. 16 has two separation chambers.

Referring to FIG. 15, fluid enters the separator 190 through a four-portinlet 191. A transition piece 192 is provided to create plug flowthrough the separation chamber 193. A transducer 40 and a reflector 194are located on opposite walls of the separation chamber. Fluid thenexits the separation chamber 193 and the separator through outlet 195.

FIG. 16 has two separation chambers 193. A system coupler 196 is placedbetween the two chambers 193 to join them together.

Acoustophoretic separation has been tested on different lines of Chinesehamster ovary (CHO) cells. In one experiment, a solution with a startingcell density of 8.09×10⁶ cells/mL, a turbidity of 1,232 NTU, and cellviability of roughly 75% was separated using a system as depicted inFIG. 15. The transducers were 2 MHz crystals, run at approximately 2.23MHz, drawing 24-28 Watts. A flow rate of 25 mL/min was used. The resultof this experiment is shown in FIG. 17.

In another experiment, a solution with a starting cell density of8.09×10⁶ cells/m L, a turbidity of 1,232 NTU, and cell viability ofroughly 75% was separated. This CHO cell line had a bi-modal particlesize distribution (at size 12 μm and 20 μm). The result is shown in FIG.18.

FIG. 17 and FIG. 18 were produced by a Beckman Coulter Cell ViabilityAnalyzer. Other tests revealed that frequencies of 1 MHz and 3 MHz werenot as efficient as 2 MHz at separating the cells from the fluid.

In other tests at a flow rate of 10 L/hr, 99% of cells were capturedwith a confirmed cell viability of more than 99%. Other tests at a flowrate of 50 mL/min (i.e. 3 L/hr) obtained a final cell density of 3×10⁶cells/mL with a viability of nearly 100% and little to no temperaturerise. In yet other tests, a 95% reduction in turbidity was obtained at aflow rate of 6 L/hr.

Further testing was performed using yeast as a simulant for CHO for thebiological applications. For these tests, at a flow rate of 15 L/hr,various frequencies were tested as well as power levels. Table 1 showsthe results of the testing.

TABLE 1 2.5″ × 4″ System results at 15 L/hr Flow rate Frequency (MHz) 30Watts 37 Watts 45 Watts 2.2211 93.9 81.4 84.0 2.2283 85.5 78.7 85.42.2356 89.1 85.8 81.0 2.243 86.7 — 79.6

In biological applications, it is contemplated that all of the parts ofthe system (e.g. the flow chamber, tubing leading to and from thebioreactor or filtering device, the sleeve containing the ultrasonictransducer and the reflector, the temperature-regulating jacket, etc.)can be separated from each other and be disposable. Avoiding centrifugesand filters allows better separation of the CHO cells without loweringthe viability of the cells. The transducers may also be driven to createrapid pressure changes to prevent or clear blockages due toagglomeration of CHO cells. The frequency of the transducers may also bevaried to obtain optimal effectiveness for a given power.

The present disclosure has been described with reference to exemplaryembodiments. Modifications and alterations will occur to others uponreading and understanding the preceding detailed description. It isintended that the present disclosure be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. A method for obtaining expanded cells, comprising: culturing cells ina fluid medium in a bioreactor to expand the cells; sending a portion ofthe fluid medium containing the expanded cells from the bioreactor to anacoustophoretic separator that comprises: a flow chamber through whichthe portion of the fluid medium flows; and an ultrasonic transducer anda reflector located opposite the ultrasonic transducer so as to producean acoustic standing wave in the flow chamber; driving the transducer toproduce the acoustic standing wave and create a concentrated flow of theexpanded cells; and further processing the concentrated flow of theexpanded cells.
 2. The method of claim 1, wherein the further processingis adapted to isolate or recover the expanded cells from theconcentrated flow.
 3. The method of claim 1, wherein the furtherprocessing is selected from the group consisting of cell washing, cellconcentration, and cell fractionation.
 4. The expanded cells produced bythe process of claim
 1. 5. A method for obtaining biomolecules,comprising: culturing cells in a fluid medium in a bioreactor, whereinthe cells produce the biomolecules; sending a portion of the fluidmedium containing the biomolecules from the bioreactor to anacoustophoretic separator that comprises: a flow chamber through whichthe portion of the fluid medium flows; and an ultrasonic transducer anda reflector located opposite the ultrasonic transducer so as to producean acoustic standing wave in the flow chamber; driving the transducer toproduce the acoustic standing wave and create a concentrated flow of thebiomolecules; and further processing the concentrated flow of thebiomolecules.
 6. The method of claim 5, wherein the further processingis adapted to isolate or recover the biomolecules from the concentratedflow.
 7. The method of claim 5, wherein the biomolecules are selectedfrom the group consisting of recombinant proteins and monoclonalantibodies.
 8. The biomolecules produced by the process of claim
 5. 9. Amethod for obtaining viruses or exosomes, comprising: culturing cells ina fluid medium in a bioreactor, wherein the cells are engineered toproduce the viruses or exosomes; sending a portion of the fluid mediumcontaining the viruses or exosomes from the bioreactor to anacoustophoretic separator that comprises: a flow chamber through whichthe portion of the fluid medium flows; and an ultrasonic transducer anda reflector located opposite the ultrasonic transducer so as to producean acoustic standing wave in the flow chamber; driving the transducer toproduce the acoustic standing wave and create a concentrated flow of theviruses or exosomes; and further processing the concentrated flow of theviruses or exosomes.
 10. The method of claim 9, wherein the furtherprocessing is adapted to isolate or recover the viruses or exosomes fromthe concentrated flow.
 11. The viruses or exosomes produced by theprocess of claim
 9. 12. A method for obtaining a desired product,comprising: sending a fluid medium containing the desired product from abioreactor to an acoustophoretic separator that comprises: a flowchamber through which the fluid medium flows; and an ultrasonictransducer and a reflector located opposite the ultrasonic transducer soas to produce an acoustic standing wave in the flow chamber; driving thetransducer to produce the acoustic standing wave and create aconcentrated flow of the desired product; and further processing theconcentrated flow of the desired product to isolate or recover thedesired product.
 13. The method of claim 12, wherein the desired productis expanded cells, biomolecules, viruses, or exosomes.
 14. A system forobtaining a desired product, comprising: a bioreactor; anacoustophoretic separator fluidly connected to and downstream of thebioreactor, the acoustophoretic separator comprising: a flow chamber forreceiving a fluid medium containing the desired product from thebioreactor; an ultrasonic transducer and a reflector located oppositethe ultrasonic transducer so as to produce an acoustic standing wave inthe flow chamber that separates the fluid medium and the desiredproduct; and a product outlet for recovering a concentrated flow of thedesired product; and at least one device for further processing of theconcentrated flow, the at least one device located downstream of theacoustophoretic separator.
 15. The system of claim 14, wherein the atleast one device for further processing is adapted to isolate or recoverthe desired product from the concentrated flow of the desired product.16. The system of claim 14, wherein the at least one device for furtherprocessing performs a process selected from the group consisting of cellwashing, cell concentration, and cell fractionation.
 17. The system ofclaim 14, wherein the desired product is expanded cells, biomolecules,viruses, or exosomes.